Right -- the most important BTO factors are those that you can't reasonably upgrade later. RAM, hard drive, even the optical drive are all basically user-serviceable parts.
The processor (since they invariably solder it on) and screen (IMHO, too delicate a job, but the brave may think otherwise) seem to me to be the important factors to deal with at purchase time.
Other stuff that PC users may consider (e.g., video card) usually aren't reasonably available factors for Mac users.